Many of today's existing gas stations were built prior to the 1980s. While gas station buildings may have been upgraded since then, often the original underground tanks used to store the fuel have not been replaced. These tanks are generally cylindrical and are composed of primarily unprotected steel with approximately 6 mm of steel thickness. Usually, the tanks are located underground and surrounded by backfill materials, or concrete ballast, to provide support for the tank. Typically, the tanks were designed to have a life of about 30 years. Further, it has been found that existing tanks have suffered external corrosive damage, in particular, pitting corrosion. In extreme cases, external corrosion can lead to penetration of the steel tank material that will cause fluid to leak from the tank to the environment. This can be hazardous, especially if the leaking fluid is flammable and combustible or poisonous. It also poses a contamination threat to nearby underground drinking water sources.
External corrosion of steel tanks takes place by localized electrochemical reactions on the surface of the steel which may be caused by soil conductivity, or by chemicals dissolved in water or moisture present in the ground. Particularly problematic is pitting external corrosion, because the corroded site tends to be quite small. As a result, chemical and electrochemical reactions occurring in the “pit” tend to produce high concentrations of corrosive ions and a high current density which accelerate external corrosion processes. Also, steel is susceptible to stress external corrosion cracking where the presence of corrosive agents at a crack can produce rapid propagation of the crack.
In order to avoid the problems associated with older steel tanks, regulations of certain states currently require secondary containment double wall construction for underground flammable and combustible liquid storage tanks. Recently, the Federal government has implemented regulations that require all states to have secondary containment tanks at locations near drinking water sources. Secondary containment construction provides secondary fluid containment to resolve environmental contamination considerations. Such secondary containment tank construction constitutes, in effect, an outer secondary containment structure that is supported about an inner primary steel tank. As a result, the interface between the inner primary tank and outer secondary containment structure defines a secondary contained (double wall) tank which provides for secondary containment of the fluid in the event a leak should develop through the wall of the inner tank.
To detect a leak in a secondary containment tank, liquid-sensing monitors are conventionally located in communication with one or more low zones in the secondary containment space (interstice) between the inner primary steel tank and the outer secondary containment structure. Such liquid-sensing monitors are generally located at the bottom of the interstice. Therefore, any leakage outwardly through a breach in the inner primary tank into the interstice, or inwardly through a breach in the secondary containment structure into the interstice, is directed by gravity toward the monitor sensors which then provide an alarm signal to surface equipment indicating the leakage.
While the design of secondary containment tanks is sound, there have been concerns with the integrity of the secondary containment structure. Specifically, the outer secondary containment structure must exhibit superior strength and durability while protecting the inner steel shells and steel heads against external corrosion. Consequently, the importance of providing a durable and reliable outer secondary containment cannot be understated.
In light of the above, it is an object of the present invention to provide a storage tank for containing flammable and combustible liquids in which a polymer (e.g. polyurea, thermoplastic, polyurethane or polyamine epoxy) jacket acting as the outer secondary containment structure encapsulates the inner primary steel tank. Another object of the present invention is to provide a tank in which the secondary containment jacket is formed from a spray-applied polymer (e.g. polyurea, thermoplastic, polyurethane or polyamine epoxy). Still another object of the present invention is to provide a method of manufacturing an underground storage tank in which a polymer (e.g. polyurea, thermoplastic, polyurethane or polyamine epoxy) is sprayed onto a foil or formable sheeting material that encapsulates the inner primary steel tank and cures to form a secondary containment jacket encapsulating the inner primary steel tank. It is another object of the present invention to provide a secondary containment (double wall) tank and method of manufacturing a tank for storing flammable and combustible liquids that is easy to implement, cost effective, simple to install, and that provides a long service life.